1. Field of the Invention
The present invention relates to a photo-sensing device used for monitoring an output of a semiconductor laser or as a photo-sensor of a light communication system.
2. Related Background Art
FIGS. 1A and 1B respectively show a top view and an X--X sectional view of a structure of a prior art photo-sensing device. As shown, in the prior art photo-sensing device, a semiconductive crystal layer 2 of a first conductivity type including a light absorption layer is laminated on a surface of a semiconductor substrate 1 of a first conductivity type having an electrode 8 of the first conductivity type formed on an underside there, and impurities are selectively diffused into the semiconductive crystal layer 2 to form a first region 3 of the second conductivity type. This is a pin photo-diode structure where the semiconductor substrate 1 is an n layer (or a p layer), the semiconductive crystal layer 2 is an i layer and the first region 3 is a p layer (or an n layer), and a photo-sensing region 10 is formed in the i layer. An electrode 6 of a second conductivity type is formed on the first region 3 on the surface of the semiconductor crystal layer 2, and the first region 3 inside the electrode 6 is covered with an anti-reflection film 5 while the semiconductive crystal layer 2 outside the electrode 6 is covered with a device protection film (i.e. passivation film) 9.
When a reverse bias is applied to the semiconductor device thus constructed, a depletion layer is created in a pn junction area in the semiconductive crystal layer 2. Thus, an electric field is developed in the depletion layer and electrons and holes generated by a light applied to the photo-sensing region 10 are directed to the first conductivity type semiconductor substrate 1 and the second conductivity type region 3, respectively, and accelerated thereby. In this manner, a photo-current is taken out and a light signal is detected.
In the structure shown in FIGS. 1A and 1B, when the light is applied to the photo-sensing region 10, light generating carriers are captured by the depletion layer and a good response characteristic is offered. However, when the light is directed to the outside of the region 10, the generated carriers reach the pn junction while they are diffused by a density gradient and are taken out as a photo-current. As a result, the response characteristic is adversely affected. FIG. 2A shows a response characteristic of the photo-sensing device. Since the movement of the carriers by the diffusion is slow, a response waveform for a light pulse includes a tail at the end as shown in FIG. 2A.
When such a photo-sensing device is used for the light communication, light emitted from an optical fiber is condensed so that it is directed to the photo-sensing region 10. However, when a portion of light leaks out of the photo-sensing region 10, it leads to the reduction of the response speed of the photo-sensing device by the reason described above. In a high speed photo-sensing device, the area of the photo-sensing region 10 is reduced to reduce a junction capacitance. As a result, a ratio of light directed to the outside of the photo-sensing region 10 increases and a diffused component which has a low response speed increases. This leads to the degradation of the response speed.
When the light emitted from a rear end plane of the semiconductor laser is sensed by the photo-sensing device to feedback-control a drive current for the semiconductor laser in order to keep the light output of the semiconductor laser at a constant level, if the light emitted from the semiconductor laser spreads to the outside of the photo-sensing region 10 of the photo-sensing device, a low response speed component is generated by the diffusion as described above. This adversely affects the feedback control.